Physical inactivity has been identified as a primary contributor of childhood obesity and related diseases, with underserved youth (minority and low-income status) at greatest risk of inactivity and its health consequences. However, physical activity (PA) interventions have attained limited-to-no sustained behavior change and thus, have rarely affected targeted physiological or anthropometric health outcomes. Despite substantial research noting the importance of social influences and goals (e.g., being with, and making friends) on PA motivation and participation, PA-based interventions have almost exclusively emphasized approaches centered on constructs related to physical ability (e.g., mastery, self-efficacy). To date, little research has focused on understanding the contributions that social goal orientations and need for connectedness make toward promoting sustained youth PA. We argue that during adolescence, when orientation towards social relationships is strongest, the desire for social connections is a primary goal of action underpinning youth PA behavior and thus, a key mechanism to target for sustained behavior change. Thus, the proposed study will address limitations in previous research by targeting the social mechanisms needed within afterschool programs (ASPs) to promote sustained increases in youth PA. Application of social development research, Achievement Goal Theory, and Self-Determination Theory provides the conceptual framework for addressing key social mechanisms: 1) Friendship (and the development of PA-based social skills); 2) Group Belonging (peer connection), and; 3) Connection with ASP Staff. Primary components of the study include Get to know you sessions aimed at providing youth guided social opportunities to foster friendship-building skills, and to promote acceptance, cooperation, and friendship affiliation, and infusing a novel socially-oriented PA curriculum within ASP free play sessions. Two afterschool organizations (N= 12 ASPs) that serve a primarily underserved population of youth have agreed to participate in the program. During year 1, staff and youth interviews, and systematic observations of ASP sites will be conducted to develop a curriculum that will be tested during an 8-week pilot in one ASP (N=40 youth). During year 2, a randomized study (3 intervention and 3 control ASPs) will be implemented to test the feasibility of the intervention approach (N=200 youth). Feasibility of our intervention should demonstrate that intervention youth report greater perceived connectedness and affiliation goal orientation toward PA from baseline to post-intervention. Observations of ASPs will test changes in the social climate from pre- to post-intervention and process evaluation will assess dose and fidelity. It is hypothesized that: 1) youth in the intervention (vs comparison) will demonstrate greater improvements in positive PA affect, cognitions, and behaviors, and BMI from baseline to post-intervention, and; 2) targeted social mechanisms will mediate the effects of the intervention on changes in PA outcomes.